hey,
First off, you could probably get 100 for your xbox on ebay, if you're old enough to sell on ebay.

I would suggest going with an entry level 35mm SLR. something like the canon rebel ti or rebel t2 will be fine, with just a kit lens. It takes film, so it's a great way to get started in photography, and I'm sure your sister will thoroughly enjoy it.

if you want to wait until I get back from camp, I will sell you my canon rebel ti and lens for a cheap price. I understand where you're coming from, and I think it's a very nice and noble thing for you to do. It's great to see families really caring about each other.